This invention relates to a device and a method for marking copper-clad laminates.
Copper-clad laminates are used as a base material in the production of printed circuit boards. The latter are made from electrically high-insulating laminated materials provided with copper cladding, i.e. a copper coating. As a general rule, the intermediate layer consists of a glass-cloth laminate with a resin matrix, e.g. of epoxy resin. The intermediate layer can also be manufactured from e.g. a glass-fibre mat, an aramide fabric or fleece, or also from a phenolic paper laminate.
A workpiece comprising a copper-clad laminate is processed in the known manner into a printed circuit by etching away parts of the copper coating so that the remaining copper surfaces form the desired conductive pattern on top of the insulating intermediate layer.
Before processing, it is desirable to mark the workpieces so that they can be clearly identified. For this purpose it is customary to mark the workpieces by stamping a symbol at some point which is not processed in the operation to manufacture the circuit board as described above. As a result, however, the copper laminate ends up permanently mechanically deformed. This deformation, which may appear for example in a marked corner of the workpiece, is highly undesirable for the subsequent processing. Furthermore, a relatively long marking device change-over time is involved whenever a change of symbol is required. It is also difficult to carry out the marking operation in such a way that the marked symbol remains visible on the intermediate layer even after the copper layer has been etched away.
The task of this invention is therefore to provide a method and a device for marking copper-clad laminates which overcome the disadvantages described above.
This task is solved according to the invention by means of the device of claim 1 and the method of claim 9.
The device according to the invention is equipped with a laser, in whose resonator there is a screen with which the intensity of the generated laser beam can be controlled. The laser beam emitted by the resonator is initially expanded by an expanding lens such as a diverging lens, for example, and then focused on the surface of the copper-clad laminate workpiece to be marked using a focusing lens such as a convergent lens.
At the focal point of the convergent lens the copper surface of the laminate is heated and vaporized by the laser beam so that the intermediate layer of the laminate is exposed. The surface of the epoxide material of the intermediate layer is carbonized by the heat, i.e. carbon compounds are formed at the surface, which blackens as a result.
This process requires a very high laser intensity, because copper is extremely heat-conducting and the necessary temperatures at the focal point are difficult to attain. This is why the focal point generated has to be very small. This is the reason why the laser beam is first expanded before being focused. With an expansion of around factor 6, for example, it is possible to obtain a focus diameter of less than 50 m. On the other hand, the laser intensity has to be appropriately dosed to avoid the intermediate layer from being burnt through together with the top copper layer. This purpose is served by the screen disposed in the resonator. By adjusting the size of the screen the intensity of the generated laser beam can be dosed very accurately.
The desired marking can therefore be created by scanning the workpiece with the laser. For this purpose the optical components of the device are advantageously grouped together to form a scanning head which moves over the workpiece surface.
The advantage of the device of the invention is that the workpiece is not deformed during the marking process, and no problems occur in this respect during further processing. Neither is there any need for any change-over operation on the marking device as simple reprogramming of the device is all that is required to change the symbol as desired. This is extremely time-saving compared with the prior art method. Furthermore, the burnt-in symbol endures even after the copper layer has been etched away.
Aside from the process for marking the intermediate layer of the laminate as described above, it is also possible to mark only the surface of the copper layer by using a lower intensity. The copper is oxidized and blackened by the heat. Given that here, too, the problem remains one of obtaining the highest possible intensities at the focal point, it is again indispensable to expand the beam and control the intensity by means of the screen.
The laser is preferably operated in a pulsed manner, i.e. pulses of light of high intensity are generated. In this case the average intensity can be determined in relation to an interval of time by controlling the pulse frequency, because the average intensity increases with the density of the sequence of pulses. The pulse frequency is preferably controlled by an audio-optical modulator disposed inside the resonator. It is also possible to control the intensity in the usual way via the laser lamp current.
In one preferred embodiment, the marking device is provided with a clamping device for fixing the area of the workpiece to be marked in the focal plane. In this case it is important that, during the marking operation, the workpiece surface to be marked is held sufficiently accurately in the focal plane to ensure that the intensity is satisfactory, and that a sharp pattern is created. In one preferred embodiment the clamping device comprises a stationary table against which the workpiece or the area to be marked rests, and a frame-likexe2x80x94i.e. provided with an openingxe2x80x94pressure plate which can be moved in relation to the table. The surface of the workpiece to be marked is rested on the table and fixed in place against the table by moving the pressure plate so that the marking process can take place through the opening in the pressure plate.
A charging device preferably serves to move the workpiece into the marking position, and for the subsequent removal of the marked workpiece. In one preferred embodiment the charging device is comprised of two roller conveyors which are disposed along a plane and transport the workpiece in two directions oriented perpendicular to each other. In this case the marking position can be advantageously defined in that the transport paths of the roller conveyors are each limited in a transport direction by stopper elements. This means that the workpiece is initially moved against a first stopper element on one roller conveyor so that the marking position is defined in this direction, after which it is moved against a second stopper element by the second roller conveyor in the direction perpendicular thereto. The marking position is thus quite clearly defined in the plane of motion.
A method for marking copper-clad laminates with the device of claim 1 is described in claim 9. Advantageous embodiments of this method are indicated in the sub-claims 10 to 13.